This invention relates to switches including circuit breakers, disconnecting switches, earthing switches and the like disposed in electric power systems, and more particularly to a switch operation monitoring apparatus for detecting an abnormal operation of an actuating mechanism for such a switch as early as possible.
Now, there is a strong tendency toward development of a technique for so-called preventive maintenance of power transmission and transformation equipments, and improvements in the reliability of power transmission and transformation, especially, interruption-free power transmission is strongly demanded. According to statistics of failures of switches such as circuit breakers, mal-operation of the circuit breakers attributable to an abnormal operation of their actuating mechanisms occurs most frequently and occupies the majority of the breaker failures. However, the actuating mechanism has a very large number of parts. Therefore, development of the so-called preventive maintenance technique for the circuit-breaker actuating mechanism is now demanded. According to the preventive maintenance technique, the movement of the movable electrode of the circuit breaker during its circuit interrupting operation, that is, the characteristic stroke of the movable electrode during the circuit interrupting operation is measured so as to detect an abnormal operation of the actuating mechanism as early as possible on the basis of whether or not a distortion is present in the measured characteristic stroke.
Many methods for measuring the characteristic stroke of the movable electrode of a circuit breaker have been proposed hitherto. For example, JP-A-No. 58-28615 discloses a method in which an optical fiber is used to electrically insulate a measuring instrument so as to permit measurement of movement of a high potential part. More precisely, according to the first prior art described above, optical marks spaced apart by a predetermined distance from each other are disposed in such a relation that ranges having different reflection factors are continuously alternated at a predetermined pitch. Light from a light source is projected onto the optical marks, and the amount of light reflected from the optical marks is converted into an electrical signal and the number of the optical marks is counted so as to measure the moving distance of a moving body.
The present invention contemplates to provide an algorithm used for deciding the presence or absence of an abnormal operation of, for example, a circuit-breaker actuating mechanism on the basis of the presence or absence of a distortion in the measured characteristic stroke of the movable electrode of the circuit breaker, as described in detail later. In the present invention, any one of known means can be used for the measurement of the characteristic stroke. A simple method for measuring such an operational characteristic is disclosed in, for example, JP-A-No. 61-203820. In the publication, the period of time required for operation of an auxiliary contact interlocked with a circuit breaker is measured. More precisely, according to the second prior art described above, a plurality of auxiliary contacts opened and closed in interlocking relation with operation of a circuit-breaker actuating system are provided, and the period of time required until the auxiliary contacts operate in response to the application of a trigger signal to the closing coil or trip coil of the circuit breaker is compared with a reference value so as to detect whether the operation of the actuating system is normal or abnormal. In other words, according to the second prior art proposing the manner of decision of an abnormal operation of the circuit-breaker actuating system, a reference operational characteristic is stored in a memory and compared with the operational characteristic actually measured during the circuit interrupting operation, and the difference between them is based to detect whether the operation of the actuating system is normal or abnormal. However, the reference operational characteristic used in the prior art represents that measured under no load, that is, at the time where no current interrupting operation is being carried out. Thus, no consideration has been given to the current interrupting operation under the on-load condition.
In the actual current interrupting operation of a circuit breaker, the characteristic stroke of its movable electrode is subjected to variations due to three factors described below even when the circuit-breaker actuating mechanism is normally operating.
The first factor causing the variation of the characteristic stroke is the actuation reaction force generated due to a pressure rise of gas in a puffer cylinder during the current interrupting operation. In a puffer type gas circuit breaker which is most sensitive to such a gas pressure rise, the range of reaction force variation amounts sometimes to a maximum of several tons. Besides the magnitude of the interrupting current, the actuation reaction force is greatly affected by the phase of the interrupting current relative to the starting time of the current interrupting operation. This first factor is greatest among the three factors causing undesirable variations of the operational characteristic of the circuit-breaker actuating mechanism during the current interrupting operation.
The second factor is a variation of the actuating force. When the actuating mechanism of the puffer type gas circuit breaker uses, for example, compressed air as the source of the actuating force, the pressure of the compressed air is controlled to lie within an allowable variation range of about .+-.10%. When the pressure of the compressed air varies within the above range, variations of the actuating force and operational characteristic of the circuit-breaker actuating mechanism are not negligible.
The third factor is a variation of the pressure of gas enclosed in the puffer type gas circuit breaker. When SF.sub.6 gas is filled in the puffer type gas circuit breaker as an arc extinguishing medium, the pressure of the SF.sub.6 gas is controlled to lie within an allowable variation range of about .+-.20%. Thus, when the pressure of the SF.sub.6 gas varies within the above range, and even when the actuating mechanism is normally operating without any trouble, the characteristic stroke of the movable electrode of the circuit breaker will vary greatly depending on the current interrupting condition. Therefore, when a prior art method is employed in which the characteristic stroke subjected to a variation is compared with the normal characteristic stroke to decide whether the operation of the actuating mechanism is normal or abnormal, a wide allowable variation range of the gas pressure must be set so as to deal with usual variations of the characteristic stroke during usual operations such as change-over of system circuits and breaking of an abnormal current. The requirement for setting such a wide allowable range of variation of the gas pressure has abstructed the desired improvement in the sensitivity of detection of an abnormal operation of the actuating mechanism.
According to another prior art method proposed to deal with the problem described above, the circuit breaker is periodically disconnected from the electric power system for the purpose of periodic inspection and is actuated under no load so as to measure the operational characteristic of the actuating mechanism. However, this periodic inspection of the circuit breaker provides a hindrance to the continuity of the service of the electric power system, while a decreased frequency of inspection impairs the reliability of electric power supply.
Thus, all of the prior art disclosures have not taken the variations of the actuating force, pressure of environmental gas and interrupting current into consideration, and the desired improvement in the sensitivity of detection of an abnormal operation of the circuit-breaker actuating mechanism has been inevitably limited.